1. Field of the Invention
The present invention relates generally to hand or catapult launched model gliders, and more particularly to structure and method providing both an efficient launch phase when the glider has relatively high kinetic energy and velocity, and an efficient and effective, but differing, glide phase when the glider has attained maximum potential energy at a low velocity.
2. Description of the Prior Art
The conventional hand or catapult launched model glider undergos two distinct and substantially incompatible phases of flight. When first launched, the glider has substantial velocity and accordingly operates most effectively with the wings at a relatively low angle of attack and a relatively low angle of incidence between the wings and pitch attitude control surface to permit the glider to gain altitude in a substantially arcuate manner. As the glider gains altitude, the speed diminishes and the lift of the wings accordingly diminishes to generate the arcuate path of travel. However, as the glider goes through the top of the arc and returns to a level configuration, the kinetic energy has largely dissipated while the potential energy of the glider is at a maximum. At this point, a relatively high angle of attack of the lifting surface as a result of an increased angle of incidence between the wing and pitch attitude control surface is required in order to provide an optimum glide. Essentially, this requires the lifting surface to be more effective at relatively low speeds as the model glides back to Earth.
Conventionally, the two phases have been compromised such that a glider launched vigorously will promptly perform an inside loop and, in most cases, terminate flight either directly or after stalling. On the other hand, a more gently launched conventional glider will gain but modest altitude and glide in a less than optimum fashion from such modest altitude.
Another approach involves gliders with spring loaded, or otherwise biased folding wings which lay back along the fuselage in the launch configuration. The glider is catapulted by, for instance, sturdy rubber bands with the aerodynamic forces holding the wings in a folded position. When the kinetic energy is expended, the glider slows and the lessened aerodynamic forces permit the wings to unfold into a glide configuration. However, it will be recognized that the transition is one from an unstable projectile to a gliding aerodynamic device thus requiring a substantial transition. This transition is not only in itself inefficient, but also contributes in most cases to a series of dives and overcorrecting climbs which provide an inefficient gliding mode.
The Jacobs U.S. Pat. No. 2,034,143 and Stark U.S. Pat. No. 2,588,941 describe a concept which is, on the surface, more appealing. This concept involves a pitch attitude control device--such as an elevator--which is positioned as a function of the glider velocity and accordingly of the glider speed produced aerodynamic force on the elevator. Thus, when first launched, the aerodynamic force at high airspeed maintains the elevator in a rather efficient configuration for a low angle of attack of the lifting surface. However, as the glider moves upward, the speed of the glider diminishes and the angle of attack is constantly increased as the air speed controlled elevator moves to a fully upward position. Accordingly, as the kinetic energy of the launch is expended, the glider is maintained in a noseup attitude with full up elevator applied as a result of the essentially zero velocity through the air. This of course is the classical stall configuration. Thus, as the effectiveness of the lifting surface fails with diminished velocity, the glider stalls and falls into a largely nose-down position and gains speed, until the lifting surface again becomes effective and, induces the glider to assume a greater and greater nose-up attitude. Again, at the time the nose or pitch configuration is optimum, the air speed tends to be relatively low and the elevator again overcorrects in an unsteady equilibrium by forcing the nose of the glider higher and higher while the airspeed gets lower and lower. Thus, classically, gliders having air speed controlled elevator surfaces display an efficient initial launch phase, but are subject to an inefficient and unstable transition into the glide phase. Rather than assuming a steady and optimum glide, such gliders tend to undergo a series of oscillations between diving and stalling in an attempt to reach equilibrium in the form of a steady glide.